2.Soil erosion,demunition of forest and desertification result from land-clearance and over-harvesting.State and explain the undesirable effects of certain agricultural practices on the ecosystem.

Agriculture,urbanization and industrialization have altered our natural landscapes dramatically.In order to satisfy the demand for food,over 40% of the usable lands on the Earth has beed used for food production including agriculture and animal grazing.On the farmlands,inappropriate agricultural practices are often adopted by people.These have caused undesirable effects on the environment,including soil erosion,deforestation,desertification and over-harvesting.

Soil Erosion

Soil erosion is one form of soil degradation by which the topsoil is removed naturally by the wearing actions of rain and wind.Soil erosion may be a slow process and occur at an unnoticed rate.It may also occur at an alarming rate and cause serious loss of topsoil every year.Soil erosion is one of the major environmental problems faced by people today.It reduces agricultural and forestry production significantly and also degrades the quality of aquatic ecosystems.

In the farmland,soil erosion is caused by inappropriate practices such as intensive ploughing and over-grazing.Soil erosion causes soil loss,leading to the reduction of crop productivity and water pollution problems.The ecological impact of soil erosion by inappropriate agricultural practices can be summarized into two major areas:destruction of agricultural farmland and water pollution.

-Destruction of agricultural farmland

Soil erosion severely affects the stability,structure and texture of soil.When the topsoil is removed,vital plant nutrients such as nitrogen,phosphorus and potassium are lost.The fertility of the soil decreases,leading to a reduced crop production.The breakdown of aggregates of soil particles due to intensive ploughing lowers the water-holding capacity of the soil,making it more susceptible to drought.If soil erosion is severe,the entire layer of topsoil and vegetation may be washed away.No more crops can be cultivated and the farmland may be abandoned.

-Water pollution

Soil is washed away by heavy rainfall into streams and water courses below the eroded farmland.It can clog drainage ditches and stream channels,and may form silt in reservoirs and harbours.Pesticides and other agricultural chemicals are carried into aquatic bodies such as streams,rivers and lakes.They may contaminate the fish spawning areas and threaten aquatic life.Also,severe erosion and water runoff on mountain slopes affect agricultural lands in the valleys below.There may be a large quantity of sediment and flooding,which further decrease agricultural productivity and speed up water pollution problems.

Deforestation

Deforestation refers to the clearance of forests and the conversion of them for non-forest uses,including agriculture,animal grazing,timbering,and urbanization.Forests may also be destroyed by natural means such as forest fires.The forests can often recover gradually after forest fires.However,deforestation due to human activities tends to be permanent.

There are two main reasons for deforestation.

As in many developing countries,the majority of subsistence farmers rely on the forest for food and also shelter.They cut down trees for growing crops.Some farmers cultivate by the combination of shifting culture and slash-and-burn agriculture.They open up an area by cutting down the trees.The woods are then piled up and burnt,and crops are grown in such ash areas.After the land has been cultivated for several years,the soil becomes infertile.Then they move and clear a new land.With the growth in population,cultivation has been the major reason for deforestation.It accounts for about 60% of tropical deforestation.

Deforestation also provides open grazing lands for livestock.Trees are always chopped down to allow grass to grow,in order to feed livestock such as sheep and cattle.After clearance,a land can provide grass for animals up to 20 years.These practices in developed countries are usually owned by foreign companies.The beef cattle,for example,are raised for export which is highly profitable.Animal grazing accounts for nearly 12% of tropical deforestation.

As forests play a vital role in the biosphere.They act as a climatic buffer by retaining moisture in the air,cooling down the air,and maintaining the balance of carbon dioxide level in the atmosphere.They lock up nutrients and maintain their availability to other organisms.They hold the soil and retain rain water.These help to prevent soil erosion,flooding,sedimentation and water pollution.Forests are also important as the habitats for a large variety of organisms.

The clearance of forests causes great impact on both the environment and humans.These include loss of nutrients availability,soil erosion,flooding,water pollution,hotter and drier weather conditions,global warming,as well as reduction in biodiversity.

Desertification result from land-clearance

Desertification is a process whereby productive farmlands in arid,semi-arid and dry subhumid areas are degraded into deserts.

Farmlands are important natural resources.Humans rely on them for food.Desertification results in the loss of farmlands,and significantly threatens the living standard and well-being of people inhabiting the areas concerned.This also leads to social problems such as environmental refugees whose lands are too eroded for cultivation or rearing livestock.

Desertification also has serious impact on the natural environment.It breaks down the fragile balance that allows plant and animal life to develop in arid,semi-arid and dry sub-humid zones.This breakdown of the equilibrum represents the start of a process which destroys the natural and stable ecosystem.

Another problem is that the results of desertification in turn speed up the natural process of desertification.The vulnerability of soil to wind and water erosion,the lowering of the water table,the impairment of the natural regeneration of vegetation,and the chemical degeneration of soils are all intensifying desertification.

Over-harvesting

Cultivated fields are relatively unstable ecosystems because all vegetations except the cultivated crop plants are removed.This has already a great impact on the soil quality.Over-harvesting further increases the impact on soil.When crops are harvested,plant tissues together with large quantities of minerals are taken away.Thus the nutrients required by the crops are rapidly depleted from the soil.If fallow periods are not sufficient and fertilizers are not applied appropriately,the soil will eventually become infertile.